The need for renewable energy sources is obvious and much effort has been directed to development of solar and wind technologies. One area of renewable energy conversion that has been relatively neglected is the use of the natural temperature variation with altitude or “lapse rate” as an energy source. The use of other naturally occurring temperature gradients such as ocean thermal energy conversion (see U.S. Pat. No. 5,582,691) have been proposed but the hostile, corrosive environment of the ocean, combined with the inherent low efficiency of small temperature differences, and the fact that the ocean thermal gradients are negative (temperature decreases with depth requiring pumps to move the water) have thus far made them impractical.
The use of the atmosphere as a thermal heat sink or source combined with external heat sources or sinks has also been considered and is practical when such external sources or (less commonly) sinks are available (see U.S. Pat. No. 5,488,828) and often require man made towers to access gravitational potential as a mechanical force agent in energy conversion.
Unlike these systems the present invention needs no external sources or sinks other than the atmosphere, and uses natural elevation changes available in mountains to both access the needed temperature difference and gravitational force to produce pressures that can be used to operate conversion machinery
Such a method of using mountain access to the atmospheric gradient and a process for extracting its energy was invented and investigated by this author at Sandia National Laboratories, operating under government contract in 1973. An analysis of that system is contained in an internal Sandia report SAND 74-0259. The results of that analysis indicated that such a system was feasible to build and, for elevation changes greater than 5000 feet, could be competitive with power plant costs at that time, with most of the cost for large systems expended on heat exchangers. The invention of that system was not patented and the analysis results were not published outside the Laboratories. Since that time the Department of Energy has waived its rights to that invention to this author, but several others have already patented similar ideas. (U.S. Pat. Nos. 3,953,971 and 4,318,275). A schematic of that original process is shown in FIG. 1. A condensable lighter-than-air vapor is vaporized in the lower heat exchanger, 1, and flows up a pipe, 2, to a condenser, 3.
Although the gas cools as it rises, it does not cool as much as the atmospheric lapse rate so that a condenser in the higher cool air region will cause the vapor to condense. The denser condensed liquid then flow down another pipe, 4, and the increased pressure is used to operate some form of machinery, 5 (hydraulic pistons or turbines for example) which may be coupled to an electric generator.
Although the system shown in FIG. 1 should work (it has been modeled but not built) it presents several practical difficulties which can be overcome. First the liquid head at the bottom of the down flow pipe would be very high for the large mountains for which this system is applicable. Hydro-turbines for those high pressures do not exist and some kind of hydraulic engine would have to be developed. Second, such high pressures in a toxic liquid present additional safety concerns, and there are very few choices of lighter than air gases that are condensable at atmospheric temperatures, with ammonia being the best candidate. Third, except for the small amount of condensate carried up by the expanding vapor, all the condensate must be produced by transfer of the whole vaporization energy to the outside air through a small temperature difference, which means the heat exchangers must be very large. These problems can be overcome or mitigated by the configuration shown in FIG. 2, which is the preferred embodiment of this invention.